As the external shapes of orthopedic surgical implants become more complex,-correspondingly sophisticated instruments are needed to prepare bone tissue to receive these implants. One such example are the instruments required to prepare the proximal femur for a total hip arthroplasty (THA) prosthesis. Other implant procedures such as total knee arthroplasty and total shoulder arthroplasty also require similar tissue preparation with milling instruments that replicate the three-dimensional geometry of the prosthesis surface.
The majority of THA implants in use today consist of three main components: the acetabular cup prosthesis, the femoral head prosthesis, and the femoral stem prosthesis. The acetabular cup prosthesis replaces the bearing cartilage in the acetabular hip socket of the pelvis. The femoral head prosthesis is a metal or ceramic ball that articulates in the acetabular cup prosthesis. It replaces the proximal spherical femoral head and the associated articular cartilage. The femoral prosthesis is typically a metal prosthesis implanted in the medullary canal of the femur. It connects the femur bone to the femoral head prosthesis and distributes the major hip loads from the acetabular socket to the femoral medullary canal.
A variety of geometrically complex femoral prosthesis designs have been developed. Originally these implants were available in a finite range of sizes and shapes. However, due to the range of anatomic variability between patients, a large inventory of implants was needed from which the surgeon would choose the best fitting prosthesis. As is inevitable with a finite selection of sizes to fit the infinitely variable anatomic structures of the human skeletal system, surgeons were typically forced to compromise their fit by selecting a prosthesis size that was either too large or too small or simply did not have the right shape to fit the patient optimally.
Consequently, modular femoral prosthesis systems have since been developed to both limit the number of parts in inventory and to also allow more intraoperative surgical sizing options. These modular systems allow the surgeon to build a prosthesis at the time of surgery that optimizes the shape of the implant to best match the unique anatomic requirements of a particular patient's proximal femur.
An assembled modular femoral prosthesis typically consists of three basic components: the proximal neck, the central body and the distal stem. A femoral prosthesis kit, available at the time of surgery, contains a range of sizes and shapes of each of these three components. Additionally, to accommodate more anatomic shaping options, these sections can be rotated at adjustable angles during assembly to optimize the shape of the final implant construct.
Although selecting the best prosthesis for a patient is critical to the success of a THA surgery, it is equally important to prepare the bone cavity to optimally fit the prosthesis. Long-term post surgical follow-up on patients has shown that the success of THA surgery is significantly influenced by the surgeon's ability to optimize the fit between the surgically prepared bone cavity and the load-bearing surface of the femoral prosthesis.
Ideally, the shape of the cavity should exactly match the shape of the external surface of the femoral prosthesis. This would allow an even distribution of the implant loads to the femoral bone, helping to prevent micro-motion between the prosthesis and the tissue. Since this micro-motion could eventually lead to loosening of the prosthesis, resulting in pain, instability and ultimately failure of the fixation, it is important to create the bone cavity so that it closely matches the implant.
Optimizing the cavity shape not only allows favorable load distribution between the prosthesis and the femur but also allows favorable tissue-to-implant apposition, so that there is more surface area contact for potential bone in-growth into the prosthesis. In the case of a non-cemented THA, this tissue-to-implant apposition allows bone tissue to eventually grow into the textured, bone in-growth surface on the prosthesis that is designed to mimic the cellular morphology of the inside of femoral bone.
Due to the variety of implant shapes that can be constructed with modular femoral prostheses, and the complexity of the anatomy of the proximal femur, it is generally not practical to precisely prepare a cavity to accept the prosthesis with conventional bone removal instrumentation. Typically, with conventional bone removal instrumentation, compromises in the shape of the cavity must be made that limit the surface area contact to allow for reasonable implant fit. At a minimum, contact in the anterior and posterior cortices, the medial cortex below the lesser trochanter, or the lateral cortex above the distal tip of the prosthesis is essential for good implant fixation. However, only contacting the implant in this area may result in a compromised fixation, unable to adequately prevent excessive relative micro-motion over the life of the implant.
The bone cavity is typically created freehand by incrementally removing small amounts of tissue with instruments such as drills, reamers, raps, and broaches. Then the surgeon intermittently tests the fit between implant and bone by inserting and removing the prosthesis and manually sculpting the cavity until the fit seems acceptable.
Another approach is to insert a reaming instrument, such as a long conical reamer or medullary drill, into the medullary cavity of the proximal femur. Then the bulk of the material in the calcar region is removed with a series of angled drills. This technique works for implants with a simple geometry, but does not work well for those with more complex curved surfaces.
Yet another approach is to use a series of broaches, each sequentially larger than the previous, that approximate the shape of the femoral prosthesis. Once the general size of the cavity is formed, the surgeon customizes it to best match the modular prosthesis geometry.
Such conventional bone removal techniques are typically not adequate to prepare the cavity for the more complex geometries associated with modular prostheses. The removal of too much tissue results in a mating surface with gaps that do not allow bone-to-implant apposition. The removal of too little tissue results in an improperly seated implant.
The texture of the cutting surface left by a bone removal instrument is also important to the long-term success of the procedure. When bone removal instruments such as broaches are used to remove relatively bulky segments of bone tissue, the resulting surface texture of the cavity is often too course for intimate implant-to-bone contact. The implant contacts the small rises between the cutting paths, leaving gaps between each area of contact.